Okay Windmils, it's Payback Time!

I was reading this upbeat article about the energetic viability of windmills. In other words, the notion that the energy a wind turbine generates while it is in use for many years is greater than the amount of energy used up building it.

https://www.treehugger.com/renewable-energy/energy-paypack-2-megawatt-wind-turbine-lasts-over-20-years-5-8-months.html

...and soon ran into this pessimistic comment below it:

NielsZoo

It takes more than a year just to cover making the steel... not counting rolling it into plate, machining, heat treat the specialty steels in the gearbox and generator alone. Figuring there's around 180 tons of steel in the tower and nacelle alone and another 50 tons minimum in the concrete base as rebar. (I'm not sure how much extra weight is added at the top in low grade iron for mass.) At 20 kW-hrs/kg that's 4.6 GW-hrs just for the steel. A 2 MW turbine at the industry average 15% of nameplate generation creates about 2.7 GW-hrs a year so just to make the steel is gonna take a year and 8 months to pay back. Now, there's usually around 1000 tons of concrete in the base of a standard 1.5~2 MW turbine. Add another 1 GW-hr for the concrete and aggregate so just the steel and concrete needs over two years to cover energy wise. This is just the raw materials for two things, no manufacturing costs, just concrete in trucks and steel billet. Then you've got glass, thermoplastics, several tons of rare earth magnets, ultra high grade precision ground gear trains also weighing tons. Many tons of copper, glass and carbon fibers and those are real energy sinks not only to make but to draw. Lets not forget all the oil as well. That's all the time I've got but it doesn't look good for something that usually dies at 8 or 9 years of its supposed 15 year lifetime.

End of quote.

Which is it then, are windmills a futile energy pit or are they a good investment that pays off more than the original energy cost? Not being an expert, I thought I'd attempt some very simple math for some of the elements of a 3MW turbine.

So starting with something simple like concrete, how much so called embodied energy is in that?

It seems concrete requires about 1.1 MJ or mega/million joules per kg, and consequently releases about one kg of CO2 into the atmosphere. Not the best start to our windmill.  I can see why some point out green energy is not all that green, but that is another issue.

A second source I found ( http://strineenvironments.com.au/factsheets/concrete-and-embodied-energy-can-using-concrete-be-carbon-neutral/ ) gives these numbers in mega joules aka MJ:

Fibre cement 4.8
Cement 5.6
Insitu Concrete 1.9
Precast tilt-up concrete 1.9
Clay bricks 2.5
Concrete blocks 1.5

I'm guessing in situ means after hauling it to the site which would give us a more realistic real world number, but it makes it a bit harder to cross compare with other which are not stated as in situ.

Back to the math. If we convert 1.1 MJ we get 0.3056 kWh per kg of concrete. Since One metric ton is 1000 kg, and each ton is now 305.5 kwh of embodied energy.

I've seen 800 tons as another estimate for the mass of the concrete foundation, obviously the bigger the wind turbine perched on top the bigger its base needs to be, assuming the same soil conditions. That 800 tons would give us 244,4 MWh for all the concrete in the foundation, but we are not adding in the energy for the many tons of re-bar or anything else like excavating the hole.

Our 3 MW turbine could be calculated at 33% capacity factor because it doesn't run at full tilt all the time. To compare with other places, wind turbines on King Island in Australia are operating at 39% capacity, because they are in a great location on a windy island. In Denmark newer, bigger and higher turbines are getting numbers in the high 40's. Still, at 33% we would need about 10 days to generate the energy embodied in the concrete foundation, without taking into account any power losses involved in sending it back into the electric grid. In other words, this is a more optimistic number.


Even if we acknowledge that not everyone has great locations for their windmills and we use the higher in situ concrete value of 1.9 MJ while dropping the capacity factor to a more pessimistic 17%, we still would only need 35 days or so to recover the energy invested. That now also includes the delivery of the concrete to the site.

How about steel? Steel takes more energy per ton to produce, and we need a lot of it. Starting with 230 tons from the comment above by NielsZoo, and using a basic steel grade with 20.1 MJ of embodied energy per kg we get 5.583 MWh per ton. For all 230 tons ( pretending it is all made of common grade steel which is not accurate ) we get 1284 MWh which our 33% CF turbine would take our about 53.5 days to generate. Our pessimistic 17% scenario would require about 100 days, assuming average wind of course.



So did I get it right? Is my math even close? At first glance it looks to me like those numbers for some of the bigger elements are not horribly of the mark and fit in with other Energy Returned On Energy Invested ( EROEI ) estimates of 18 to 1 or so for wind turbines. I can imagine adding in a multitude of components, some bigger than others, but many getting smaller and smaller as you narrow down the total amount of energy that goes into building, transporting and erecting a windmill.

Longer term it could very well be possible to refurbish windmills and keep them producing after their original intended lifespan. This is done with other types of expensive equipment, from locomotives to bridges and ships. Windmills are not the most complex of machines. After twenty years bearings could be replaced, the same with gears, seals and wiring. The concrete foundation is extremely durable and the mast can be maintained for a long time with well established methods to control corrosion. The longer they are kept functioning the better of an investment they become.

The issue with windmills doesn't seem to be the EROEI, but rather their intermittency of electricity production. Yes the price of the electricity they produce sure is cheap, but it is of the lowest grade because you have no idea when you are going to get it. That takes more than a little getting used to, and adds far more to the cost of the electricity coming from windmills that anything else.


That's part of why I'm not the greatest fan of windmills, no pun intended, but I don't hate them either. In a way I see them as among the best of a bad lot, and there doesn't seem to be much in the way of alternatives.